Electrodialysis of fixer containing solutions

ABSTRACT

Silver and halide are removed from a solution containing a fixing agent by passing the solution through the cathode chamber of an electrodialysis cell. The electrodialysis cell has an anionic semipermeable membrane separating the anode and cathode and further contains a solution containing an electrode active oxidizable species in the anode chamber.

United States Patent 1 Fisch et al.

ELECTRODIALYSIS 0F FIXER CONTAINING SOLUTIONS Inventors: Richard S. Fisch; Norman Newman,

both of St. Paul, Minn.

Minnesota Mining & Manufacturing Company, St. Paul, Minn.

Filed: Aug. 24, 1973 Appl. N0.: 391,509

Published under the Trial Voluntary Protest Program on January 28, 1975 as document no. B 391,509.

Assignee:

US. Cl. 204/109; 96/60 BF lnt. Cl. CZSC l/12 Field of Search 204/ I09; 96/60 BF;

[ Dec. 9, 1975 {56] References Cited UNITED STATES PATENTS 3,634,088 I/l972 Cooley 96/60 BF 3,700,450 l0/l972 Cooley 96/60 R Primary Examiner-R. L. Andrews Attorney, Agent, or FirmAlexander, Sell, Steldt & DeLaHunt 2 Claims, No Drawings ELECTRODIALYSIS OF FIXER CONTAINING SOLUTIONS In the normal photographic processing sequence there is invariably a fixing step in which silver is removed from the photographic element. This is generally accomplished by the formation of a water-soluble silver thiosulfate complex. In most cases, at least a part of the silver removed from the element comes from an insoluble silver halide salt, so that the hypo containing bath contains not only silver but halide as well. Recovery of the silver from this solution has permitted the reuse of these hypo containing solutions. Although a variety of methods have been used for the removal of the silver, the most satisfactory method has been the use of electrolytic deposition techniques (e.g., US. Pat. No. 1,857,507) in which the complexed silver ions are removed by conversion to free silver metal. Even under the conditions of the most effective removal of silver, continued re-use of the hypo containing bath eventually fails. This is due to the build up of halide ions and their consequent inhibition of fixation. Some techniques have been proposed for the removal of these halide ions so that the useful life of these hypo solutions is extended. Such techniques (e.g., ion exchange resins) are expensive, and demand much labor to maintain a system in continuing operation.

We have now found a simple effective means for simultaneously removing the silver and halide ions from a bath containing a fixing agent (e.g., sodium, ammonium or potassium thiosulfate). These baths include fixer baths, bleach-fixes and monobaths (developer and fixer in the same solution).

We have found that the use of an electrodialysis cell separated into two compartments by a semipermeable membrane effectively removes both silver and halide from hypo containing baths, allowing their continued re-use with consequent reduction in waste disposal problems. The hypo containing solution is passed through the compartment with the cathode electrode upon which the silver is deposited as silver metal. The second compartment contains the anode electrode, and through this flows a receiving solution. The semipermeable membrane is an anionic membrane permeable to halide ions but impermeable to cationic species and multiply charged anions. The receiving solution is an aqueous solution preferably dilute and containing a species which can be oxidized providing a driving force for the transfer of halide across the membrane. Such a solution could be one containing a soluble ferrous salt, sulfite, or stannous salt or other common reducing agent which can be oxidized at the surface of the anode. In accordance with the need to reduce waste disposal problems, the receiving solution which is eventually discarded should preferably contain innocuous materials such as ferrous sulfate, sodium sulfite, ascorbic acid or sugar-type derivatives of ascorbic acid and the stereoisomers and diastereoisomers of these acids. The derivatives of ascorbic acid useful in the practice of this invention are disclosed in U.S. appl. Ser. No. 391,507, filed concurrently with this application, attorney's docket No. 28,426.

One of the advantages of this system is the fact that both silver and bromide are simultaneously removed from the fixer solution. Although there is some determinable amount of silver halide in solution or suspension, which one would expect to be removed by electrodialysis, the major portion of the silver exists as a complexed salt of thiosulfate. In an ordinary electrolytic process, the two species from the complexed salt, the silver complex and the halide, would be expected to associate themselves with differing electrodes but the hypo and halide would remain in the same solution. However, the fixer could not be continually reused because of the remaining halide and its buildup. This problem is obviated with the present process.

When the electrodialysis system is used in conjunction with metal chelate developers, the wash water may be passed through the anode chamber as the receiving solution.

EXAMPLE An electrodialysis cell of the following description was used in this example. A semipermeable membrane (Ionics l l l-BZL1065) separated the cell into two chambers, one chamber containing the anode and the other chamber, the cathode. Two openings were provided for each chamber as inlet and outlet passages for liquid streams. The flow of the fixer solution was directed into the cathode chamber, and the receiving solution was directed into the anode chamber.

A hypo containing bath containing Na thiosulfate .SM Ag+ 0.] Molar l liter Br- 0.] Molar pH 5.5

was passed through the apparatus at 10 volts 0.10 amps for 2 hours.

A receiving solution comprising FeSO, 0.] Molar 1 liter was passed through the anode chamber.

After 2 hours the silver measured in the hypo solution was 0.038 M. The Br in the receiving solution was measured to be 0.051 Molar. The data indicate that 0.062 Moles Ag was removed from solution (62%) and 0.051 Moles Br transferred to the receiving solution (51%) from the hypo containing bath.

The receiving solution in the practice of this invention must, as previously indicate, contain an oxidizable species in the solution. The oxidizable species must also be an electrode active oxidizable species which means that the species is capable of having the oxidation reaction (electron transfer) occur at an electrode.

The ascorbic acid materials useful in the practice of this invention as oxidizable species are ascorbic acid, its sugar-type derivatives and stereoisomers and diastereoisomers of the acid and its derivatives. The ascorbic acids, including the sugar-type derivatives of ascorbic acid as well as ascorbic acid, may be represented by the generic structural formula:

OH OH R-CH-C'C-C X 0 wherein X is an oxygen atom or imino group, R is any 3. an anionic semipermeable membrane separating the anode and cathode and forming two chambers within the cell, and

4. a power source connected to the anode and cathq ode, passing an aqueous solution of an electrode active oxidizable species through the anode chamher. and with the power source applying a voltage differential across the anode and cathode.

2. The process of claim 1 wherein the oxidizable species is selected from a soluble ferrous salt, soluble sulfite, soluble stannous salts, ascorbic acid, or derivatives of ascorbic acid. 

1. AN ANODE
 1. A PROCESS FOR THE REMOVAL OF SILVER AND HALIDE FROM A THIOSULFATE SOLUTION COMPRISING THIOSULFATE, SILVER IONS, AND HALIDE IONS, SAID PROCES COMPRISING PASSING THE THIOSULFATE CONTAINING SOLUTION THROUGH THE CATHODE CHAMBER OF AN ELECTRODIALYSIS CELL WHICH COMPRISES:
 2. A CATHODE
 2. The process of claim 1 wherein the oxidizable species is selected from a soluble ferrous salt, soluble sulfite, soluble stannous salts, ascorbic acid, or derivatives of ascorbic acid.
 2. a cathode
 3. an anionic semipermeable membrane separating the anode and cathode and forming two chambers within the cell, and
 3. AN ANIONIC SEMIPERMEABLE MEMBRANE SEPARATING THE ANODE AND CATHODE AND FORMING TWO CHAMBERS WITHIN THE CELL, AND
 4. A POWER SOURCE CONNECTED TO THE ANODE AND CATHODE, PASSING AN AQUEOUS SOLUTION OF AN ELECTRODE ACTIVE OXIDIZABLE SPECIES THROUGH THE ANODE CHAMBER, WITH THE POWER SOURCE APPLYING A VOLTAGE DIFFERENTIAL ACROSS THE ANODE AND CATHODE.
 4. a power source connected to the anode and cathode, passing an aqueous solution of an electrode active oxidizable species through the anode chamber, and with the power source applying a foltage differential across the anode and cathode. 